Goat Island is just north of the village of Matakana and another ideal spot to spend a summer's day. It was New Zealand's first fully protected marine reserve so you don't have to go very far offshore to be surrounded by schools of fish. If you prefer diving you can do that too— either with your own gear or hire some from Goat Island Dive—and another option which I personally love is to kayak around the entire island. There are caves in the rocks to paddle into; just make sure you don't get too close to the walls...
Photo: gorentals.co.nz.